1. Field of the Invention
This invention relates generally to optical metrology methods and apparatus and, more particularly, relates to method and apparatus for using light to examine and characterize integrated circuit substrates and devices.
2. Brief Description of Background
The increasing sophistication of semiconductor technology has resulted in a significant shift away from aluminum as the dominant metal in multi-level metallization processes. Copper has been proven to be a suitable metal to use for the metallization, in limited production volumes, and is likely to increasingly become the metal of choice. The differences between aluminum and copper present unique challenges and opportunities for manufacturers of metrology systems, and the manufacturer of metrology equipment that can establish the greatest value early in the implementation of copper based metallization processes is likely to dominate that segment of the market.
Presently, there are several methods for depositing thin films of copper for back end of the line (BEOL) metallization processes. The dominant method currently involves depositing a seed layer of copper on top of a barrier metal such as tantalum, then electroplating a thick layer of copper on top of the seed layer. Once deposited, it is important to be able to verify that the deposited copper is within specifications. What is needed is a way to measure e the thickness of copper films.
The traditional method for measuring the thickness of copper films is the four-point probe method. With this technique, originally developed in the 1950s, an array of four pointed probes arranged in a straight line is pressed into the conductive film. Current is applied to one outer probe, and returned via the opposite outer probe. A measurement of the volt age between the middle two probes is combined with the amount of current and knowledge of the bulk resistivity of the film to determine the thickness of the film. However, this method requires that the probes penetrate the surface of the conductive layer. Doing so causes scratches, and can also cause small amounts of particulates to form that can cause defects elsewhere on the wafer. Additionally, this method requires a priori knowledge of the bulk resistivity of the film. The bulk resistivity depends in part on the grain structure and orientation of the grains in the metal, and a problem that has challenged process engineers working on copper metallization processes is that the grain structure and orientation of the grains in copper changes as a function of time, even if the metallized wafers are left at room temperature. What is thus needed is a non-contact way to measure metal films.
One known type of optical metrology system used for measuring thickness employs a process that involves focusing two pulsed laser beams onto a spot of approximately 1 to 50 microns on a sample. One beam delivers a pulse of energy that induces stress near the surface of the sample. The stress propagates as an acoustic wave away from the sample surface and is partially reflected back towards the surface when it encounters the film/substrate interface or an interface with an underlying film. The second light pulse, time-delayed relative to the first, senses the stress when it reaches the surface via the stress-induced change in the sample""s optical properties (e.g. optical reflectance). Analysis of the change in reflectance as a function of time yields accurate measurements of the thickness of each layer in the sample. However, surface roughness causes the measured thicknesses to become position sensitive, i.e. , shifting the beams just a few microns can produce significant changes in the measured thicknesses. What is needed is a way to accurately measure film thicknesses where the surface is rough.
Increasing the area over which measurements are made by defocusing one or both beams allows the signal to be averaged over a wider area. For modest amounts of surface roughness, defocusing can work. However, defocusing increases the measurement area, and since the acoustic signal decreases in proportion to the square of the beam diameter, the signal to noise ratio (S/N) degrades accordingly. What is needed is a way to increase measurement area without excessively degrading the acoustic signal.
Increasing the number of measurements within a small neighborhood of a desired measurement site and averaging the thickness results allows the effect of surface roughness to be averaged. However, such an approach significantly increases the total measurement time (by the number of measurements per site), which can significantly reduce throughput. A related approach involves increasing the number of measurements in the neighborhood of a measurement site, but averaging the raw data prior to determining layer thicknesses. Although this approach significantly reduces the measurement time compared to the above-mentioned approach, it still requires a significant amount of time to acquire the raw data. What is needed is a way to overcome measurement sensitivity to spatial variations without adversely affecting measurement throughput.
Furthermore, when using existing metrology systems measurements in any given location depend on the surface roughness of the film. Thus, measurements made in approximately the same region will differ solely due to the roughness of the surface. Multiple measurements decrease throughput, and if only a few measurements are made it is unclear which measurement value best represents the film being measured. What is needed is a way to increase the repeatability of measurements made on rough surfaces.
Vibrating the sample is another approach that can be used to overcome measurement sensitivity to spatial variations. In this approach, a mechanical displacement transducer, e.g., a piezoelectric transducer, is attached to the mechanical assembly on which the sample is placed. Energizing the displacement transducer appropriately causes the sample and the associated measurement location to move. However, there is a relatively low upper limit to the amplitude of vibrations to which a sample can be driven due to the mass of the sample and the sample mounting stage. For displacement amplitudes in the range of tens to hundreds of microns, the measurement frequency is limited to less than 10 Hz by the mass, shape, and resonance frequencies of the sample and sample holder. In addition, mechanically vibrating the sample causes the rest of the measurement stage to vibrate, thus introducing noise to the rest of the system. What is needed is a way to overcome measurement sensitivity to spatial variations without adding mechanical noise to the system.
Increasing the laser beam intensities to enhance the signal to noise ratio causes local heating as a result of optical absorption of successive laser pulses, which in turn causes local annealing. Ideally, the measurement is non-destructive, and to the extent that thermal loading affects the wafer being tested, the measurement is not non-destructive. What is needed is a way to decrease thermal loading. If the pump and probe laser beams can be diverted to a different spot on the sample in a time frame shorter than the occurrence of significant thermal build-up, the non-destructiveness of the measurements can be maintained. For samples having low thermal conductivity or thick layers underneath, this time scale may be on the order of 10-100 laser pulses, thus for a laser with 80 MHz repetition rate a deflection frequency in the range of 1-10 MHz might be necessary to avoid thermal build up.
Photoacoustic systems for measuring the thickness of layers in a film stack on a substrate are well known. Also well known are the use of electro-optic modulators (EOMs) and acousto-optic modulators (AOMs), as well as piezoelectric actuators are also known in the art.
The following U.S. Patents are of interest to the teachings of this invention.
U.S. Pat. No. 6,008,906, xe2x80x9cOptical method for the characterization of the electrical properties of semiconductors and insulating filmsxe2x80x9d, describes a method for characterizing a sample includes the steps of (a) providing a semiconductor material; (b) applying at least one of an electric field, a pulsed or cw light source, a change in temperature and/or a change in pump pulse intensity to the semiconductor material; (c) absorbing pump light pulses in a portion of the semiconductor material and measuring changes in optical constants as indicated by probe light pulses applied at some time t following the absorption of the pump light pulses; and (e) associating a measured change in the optical constants with at least one of a surface charge, dopant concentration, trap density, or minority carrier lifetime.
U.S. Pat. No. 4,710,030, xe2x80x9cOptical generator and detector of stress pulsesxe2x80x9d, describes an optical stress pulse generation and detection system for non-destructively measuring physical properties of a sample, which uses a pump beam having short duration radiation pulses having an intensity and at least one wavelength selected to non-destructively generate a stress pulse in a sample and directs the non-destructive pump beam to a surface of the sample to generate the stress pulse. The optical stress pulse generation and detection system also uses a probe radiation beam and guides the probe beam to a location at the sample to intercept the stress pulse. The change in optical constants induced by the stress pulse is detected by observing the probe beam after it intercepts the stress pulse.
U.S. Pat. No. 5,379,109, xe2x80x9cMethod and apparatus for non-destructively measuring local resistivity of semiconductorsxe2x80x9d, describes an apparatus for non-destructively measuring resistivity of a semiconductor, such as InP, and includes light sources for illuminating a pre-selected portion of the semiconductor with first and second light beams, each of a pre-selected single wavelength, the first light beam operating to excite the semiconductor by photo injecting carriers, and the second light beam bombarding the local portion of the semiconductor with a pre-selected photon energy. The apparatus measures a fractional change in reflectance of the second light beam responsive to the first light beam, and records this fractional change in reflectance for various values of photon energy of the second light beam, to generate a photoreflectance line-shape. The photoreflectance line-shape is used to calculate a photoreflectance line-shape phase angle, which is used to determine the resistivity of the pre-selected portion of the semiconductor.
U.S. Pat. No. 5,546,811, xe2x80x9cOptical measurements of stress in thin film materialsxe2x80x9d, describes a method for determining the residual stress in an unsupported region of a thin film. The method includes the steps of (a) optically exciting the film with a spatially and temporally varying optical excitation field to launch counter-propagating acoustic modes along at least one wave vector; (b) diffracting a portion of an optical probe field off the excited acoustic modes to generate a time-dependent signal field at the excitation wave vector; (c) detecting the signal field to generate a time-dependent, light-induced signal; (d) analyzing the light-induced signal to determine the frequencies of the acoustic modes; (e) partially determining the dispersion of at least one mode; and, (f) comparing the measured dispersion to that calculated using a mathematical model to allow the residual stress properties of the unsupported region of the film to be determined.
U.S. Pat. No. 5,693,938 xe2x80x9cOptical probe microscope having a fiber optic tip that receives both a dither motion and a scanning motion, for nondestructive metrology of large sample surfacesxe2x80x9d, describes an optical probe microscope that includes an optical fiber oriented in a vertical direction. The fiber has a tip that emits light onto a horizontal surface of a sample to be measured. This surface can have both desired and undesired departures from planarity. An electromechanical device for imparting dither motion to the fiber tip is superposed on another electromechanical device for imparting two-dimensional horizontal scanning motion to the fiber tip. The dither motion has a much higher frequency than that of the scanning motion. Between successive scans, another device moves the sample itself from one horizontal position to another. A microscope receives the optical radiation either transmitted or reflected by the sample surface. The microscope forms a (magnified) image of this received optical radiation on the surface of an optical image position detector. The surface of this detector has a relatively large area compared with that of the (magnified) image. The resulting electrical signal developed by the detector provides desired information concerning the scanning position of the fiber tip. Also, this electrical signal is processed and fed back to a vertical pusher that maintains desirably constant the distance of the fiber tip from the sample surface.
U.S. Pat. No. 5,748,318 xe2x80x9cImproved Optical Stress Generator And Detectorxe2x80x9d describes a system for the characterization of thin films and interfaces between thin films through measurements of their mechanical and thermal properties. The system can optically induce stress pulses in a film and optically measure the stress pulses propagating within the film. Change in optical transmission or reflection is measured and analyzed to give information about the ultrasonic waves that are produced in the structure.
A first distinction between the teachings of this invention and the prior art known to the inventor is in providing an apparatus for measuring the thickness of layers in a thin film stack using a photoacoustic measurement system that includes a dither EOM to sweep the measurement spot in an area about a measurement site, and to obtain a signal representing an average for the film stack under the area.
A second distinction between the teachings of this invention and the prior art is in providing an apparatus for measuring the thickness of layers in a thin film stack using a photoacoustic measurement system that includes a dither AOM to sweep the measurement spot in an area about a measurement site, and to obtain a signal representing an average for the film stack under the area.
A third distinction between the teachings of this invention and the prior art is in providing an apparatus for measuring the thickness of layers in a thin film stack using a photoacoustic measurement system that includes a piezoelectric dither assembly to sweep the measurement spot in an area about a measurement site, and to obtain a signal representing an average for the film stack under the area.
A fourth distinction between the teachings of this invention and the prior art is method for determining the average film thickness of an opaque film in an area about a measurement site on a wafer. The method includes steps of: (a) bringing the optical assembly of the measurement system into focus; (b) aligning the beam spot with a measurement site; (c) turning on a dither EOM according to a pre-determined plan; (d) making a measurement; (e) recording the measurement data; and (f) analyzing the measurement data to determine an average film thickness in the measurement area.
A fifth distinction between the teachings of this invention and the prior art is a method for determining the average film thickness of an opaque film in an area about a measurement site on a wafer is a method that includes the steps of: (a) bringing the optical assembly of the measurement system into focus; (b) aligning the beam spot with a measurement site; (c) turning on a dither AOM according to a pre-determined plan; (d) making a measurement; (e) recording the measurement data; and (f) analyzing the measurement data to determine an average film thickness in the measurement area.
A sixth distinction between the teachings of this invention and the prior art is a method for determining the average film thickness of an opaque film in an area about a measurement site on a wafer, where the method includes the steps of: (a) bringing the optical assembly of the measurement system into focus; (b) aligning the beam spot with a measurement site; (c) turning on a piezo-electric dither assembly according to a pre-determined plan; (d) making a measurement; (e) recording the measurement data; and (f) analyzing the measurement data to determine an average film thickness in the measurement area.
The teachings of this invention provide for the use of an electro-optic deflector or an acousto-optic deflector to sweep the measurement spot on the sample surface to decrease thermal loading.